1. Field of the Invention
The present invention relates to an image processing apparatus having a luminance-type gamma correction circuit capable of changing a nonlinear characteristic, and an image processing method therefor, and more particularly, to an image processing apparatus in which a variation of saturation occurring when the nonlinear characteristic of the luminance-type gamma correction circuit is changed is suppressed and an image processing method therefor.
2. Description of the Related Art
In an image pickup apparatus such as a digital video camera, image processing is performed for an output signal from an image pickup element to generate a video, then the video is outputted to a monitor or the like. Since the monitor has a gamma characteristic, an image processor of performing the image processing includes a gamma correction circuit for correcting the gamma characteristic of the monitor. Here, when correction of an inverse gamma characteristic of the monitor is performed with respect to a luminance signal, the luminance signal of the monitor output video generally linearly varies with respect to an incident light.
However, in a state with an appropriate exposure, when the correction is performed with the inverse gamma characteristic of the monitor, a dynamic range becomes low, and over exposure occurs in relatively-low luminance parts of a high-luminance portion. This does not cause a problem for a low-contrast subject, but does cause a problem of increasing over exposure parts in a video of the high-luminance portion when a contrast becomes high. Consequently, there is known a method of setting a slope of a nonlinear characteristic in the gamma correction of the high-luminance portion to be smaller than that of the inverse gamma characteristic of the monitor, and furthermore giving a gradation to a signal of the high-luminance portion. According to this method, while a gradation expression in the high-luminance portion becomes low, a video having higher luminance can be expressed.
However, if a color-difference signal is generated by using a signal processed with such a nonlinear characteristic, a shift occurs with respect to the inverse gamma characteristic of the monitor, and thus a hue shift occurs. Consequently, there is a proposed approach in which gamma correction circuits for luminance and for color are separately provided, and a luminance signal and a color signal are generated from signals corrected with different nonlinear characteristics.
Moreover, an approach for generating an output signal obtained by the gamma correction of the color signal, with the number of bits larger than that of an output signal obtained by the gamma correction of the luminance signal has been invented (see Japanese Laid-Open Patent Publication (Kokai) No. 2001-45308). In this method, since the color signal is generated with a signal that is not suppressed in the high-luminance portion, a color shift or decolorization in the high-luminance portion is improved.
Furthermore, there is a known approach for detecting a signal amount of the high-luminance portion in the video, and changing gradation of the high-luminance portion depending on the detected signal amount. In this approach, the nonlinear characteristic for the gamma correction for the luminance is varied depending on the signal amount of the high-luminance portion. For example, if the signal amount of the high-luminance portion is large, the slope of the high-luminance portion is decreased in a property of the gamma correction. However, instead of it, the gradation expression is performed for a further higher-luminance portion.
In the above described approach for changing the gradation of the high-luminance portion depending on the signal amount of the high-luminance portion in the video, the saturation may vary along with the change of the nonlinear characteristic. Even for the same subject, the variation of the saturation can occur depending on the signal amount of the high-luminance portion.